Chapter 9 Section 2 The Structure of DNA
A Winding Staircase • James Watson and Francis Crick determined that a DNA molecule is a double helix – two strands twisted around each other, like a spiral staircase.
A winding staircase • Nucleotides are the subunits that make up DNA. • Each nucleotide is made of three parts: • 5 Carbon sugar molecule • Phosphate group • Nitrogen containing base
Nucleotide Draw This Nucleotide
Nucleotide • The five carbon sugar in DNA nucleotides is called deoxyribose. • The nitrogen base in a nucleotide can be either a bulky, double-ring purine, or a smaller, single-ring pyrimidine.
Chargaff’s Observations • In 1949, Erwin Chargaff observed that for each organism he studied, the amount of adenine always equaled the amount of thymine. A = T
Chargaff’s Observations • Likewise, the amount of guanine always equaled the amount of cytosine. G=C
Chargaff’s Observations • However, the amount of adenine + thymine and of guanine + cytosine varied between different organisms. C=42% G=42% C=51% G=51% A=58% T=58% A=49% T=49%
Wilkins and Franklin’s Photographs • By analyzing the complex patterns on X-ray diffraction photo, scientists can determine the structure of the molecule. • In 1952, Maurice Wilkins and Rosalind Franklin developed high quality x-ray diffraction photographs of strands of DNA
Wilkins and Franklin’s Photographs • These photographs suggested that the DNA molecule resembled a tightly coiled helix and was composed of two or three chains of nucleotides.
Watson and Crick’s experiment • Franklin’s image was such a key clue for Watson and Crick that it only took them a few weeks to figure out the structure of DNA after they saw it.
Watson and Crick’s experiment • In 1953, Watson and Crick built a model of DNA with the configuration of a double helix, a “spiral staircase” of two strands of nucleotides twisting around a central axis.
Watson and Crick’s experiment • The double helical model of DNA takes into account Chargoff’s observations and the patterns on Franklin’s X-ray diffraction. • In 1962, Watson, Crick and Wilkins received the Nobel Peace Prize – Rosalind Franklin did not share in the award because she died in 1958.
Pairing between Bases • An adenine on one strand always pairs with a thymine on the opposite strand, and a cytosine on one strand always pairs with a guanine on the opposite strand.
Pairing between Bases • These base pairing rules are supported by Chargaff’s observations. • The strictness of base pairing results in two strands that contain complimentary base pairs.